1. Field of the Invention
This invention relates to novel glass compositions and glass-ceramic materials made by heating such glass compositions in order to crystallize them and further, to methods for making the glass compositions and glass-ceramic materials.
More particularly, the present invention relates to the glass compositions which belong basically to the five-component system comprising of P.sub.2 O.sub.5, SiO.sub.2, B.sub.2 O.sub.3, Al.sub.2 O.sub.3 and M.sup.2+ O (wherein M.sup.2+ is an alkaline earth metal), and which are characterized by their relatively high contents of calcium phosphates and the substantial absence of alkali metal oxides therein, and which further have the advantage that they can be molded at relatively low temperatures notwithstanding their high transformation temperatures.
Furthermore, the invention relates to novel glass compositions which can be readily transformed into glass-ceramic materials having higher mechanical strength by thermal treatments for crystallization, and that can serve not only as a bioglass, a bioglass-ceramic, etc., but also as useful materials having various other purposes.
2. Description of the Prior Art
Japanese Patent Publication No. 12,576/61 discloses a method of making semicrystalline ceramics of a SiO.sub.2 -Ca.sub.2 P.sub.2 O.sub.7 -Al.sub.2 O.sub.3 type system, which includes the steps, in sequence, of melting a glass composition comprising from 30 to 48 wt% silica, from 34 to 54 wt% calcium pyrophosphate, and from 15 to 21 wt% alumina, wherein the ratio of the calcium pyrophosphate content to the alumina content should be adjusted to at least 1.85/1, and the above-described components should comprise at least 95 wt% of the composition; cooling the molten glass composition to make a glass product; heating the glass product to a temperature of from 850.degree. C. to 1,050.degree. C. at a heating rate of about 300.degree. C./hour or less; and maintaining the temperature of the glass product till the linear coefficient of thermal expansion of the glass is increased beyond 90.times.10.sup.-7 cm/cm.degree.C. (at 0.degree. to 300.degree. C.). According to this method, semicrystalline ceramic products having attributes of bone china can be obtained.
In such a glass composition as described above, which contains, converted to an oxide basis, from 19 to 30 wt% P.sub.2 O.sub.5, from 30 to 48 wt% SiO.sub.2, from 15 to 21 wt% Al.sub.2 O.sub.3, and from 15 to 24 wt% CaO, to melt the glass batch at about 1,550.degree. C. for at least 16 hours is required to obtain homogeneous glass. Accordingly, the above-described method suffers the disadvantage from an industrial point of view that a high temperature and a long heating time are required to melt the glass.
Japanese Patent Application (OPI) No. 73019/76 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) discloses glass-ceramic materials of the calcium phosphate series, wherein glass compositions which have atomic ratios of calcium to phosphorus adjusted to 1.7/1 or less, and a phosphoric acid content of 10 wt% or more (calculated in a form of P.sub.2 O.sub.5) are melted and then submitted to a thermal treatment for crystallization, resulting in transformation to a glass ceramic material having crystallinities comprising from 5 to 100 wt% of the final glass-ceramic material.
In these compositions, contents, calculated in the form of oxide, P.sub.2 O.sub.5, is 10 wt% or more, and the calcium content, calculated in a form of CaO satisfy the relation CaO content (wt).ltoreq.1.343.P.sub.2 O.sub.5 content (wt). The maximum of the CaO/P.sub.2 O.sub.5 ratio (i.e., 1.343/1) is situated between the CaO/P.sub.2 O.sub.5 ratio of calcium orthophosphate Ca.sub.3 (PO.sub.4).sub.2 (=1.185/1) and that of tetracalcium phosphate 4CaO.P.sub.2 O.sub.5 (=1.580/1), and somewhat larger than that of hydroxy apatite. Since the melting point of calcium orthophosphate is about 1,730.degree. C., a very high temperature is required for the vitrification thereof and consequently, the vitrification is difficult. In addition, in the case of a sample composition shown in an example having a CaO/P.sub.2 O.sub.5 ratio of about 0.790/1, which value corresponds to the CaO/P.sub.2 O.sub.5 ratio of calcium pyrophosphate, the glass obtained by melting the batch in a scale of 1 kg portion and then cooling it slowly causes a surface devitrification phenomena. Therefore, it is difficult to cool a large quantity of molten glass in order that the glass to be obtained may not be devitrified.
Further, Japanese Patent Publication No. 8,970/76 disclosed glass-ceramic materials and methods of making them, wherein glass compositions containing as main components from 20 to 60 wt% SiO.sub.2, from 5 to 40 wt% P.sub.2 O.sub.5, from 2.7 to 20 wt% Na.sub.2 O, from 0.4 to 20 wt% K.sub.2 O, from 2.9 to 30 wt% MgO, and from 5 to 40 wt% CaO and, optionally, from 0.05 to 3 wt% fluorine, are melted, and, after being cooled, are reheated in order to crystallize and transform them into glass-ceramic materials.
The thus-obtained glass-ceramics are useful for biological implanting materials, with respect to which attention must be given to the ratio of the univalent alkali ions Na.sup.+ /K.sup.+, because of leaching of these ions from the ceramic materials when the materials are implanted in animal bodies, and the fact that changes in the Na.sup.+ /K.sup.+ ratio can have a great influence upon the bodies of animals. Such being the case, the relation between the Na.sup.+ /K.sup.+ ratio in the glass composition and the increment of the Na.sup.+ /K.sup.+ ratio in a leached solution was examined by a leaching test in boiled Ringer's solution, and the Na.sup.+ /K.sup.+ ratio was found to be controlled within the scope of the above-described restrictions upon the compositions. Moreover, some other findings are also described therein, for instance: (1) the crystalline phase formed by the thermal treatment for crystallization was identified to be carbonate apatite; (2) the addition of fluorine was very effective because it acted as a nucleating agent for the crystallization; and (3) the mechanical strength of the glass-ceramic obtained could be enhanced by the replacement of sodium ions present at the surface layer by the treatment in a molten potassium salt, i.e., by an ion exchange treatment.
When alkaline oxides are not contained in the above-described compositions in substantial amounts, it is thought that influences upon blood in living bodies is neglibly small.
Alumino phosphate glasses comprising a P.sub.2 O.sub.5 -B.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -M.sup.2+ O (wherein M is Mg or Ca) system, which are suitably used for forming the inner tubes of sodium discharge lamps because of their small coefficients of thermal expansion, are described in A. E. Dale & J. E. Stanworth, Journal of Society of Glass Technology, Volume 35, pages 185-192 (1951). The content ranges of the oxides constituting the above-described glass compositions are as follows: P.sub.2 O.sub.5 is from about 15 to 31 wt%; B.sub.2 O.sub.3 is from about 14 to 44 wt%, Al.sub.2 O.sub.3 is from about 15 to 28 wt%, MgO is from about 11 to 17 wt%, CaO is from about 0 to 6 wt%; and, additionally, SiO.sub.2, which is introduced into the glass compositions as a contaminant through the corrosion of refractory used in the melting process is about 7 wt% or less. Since these alumino phosphate glasses retain a linear coefficient of thermal expansion at a low level of about 50.times.10.sup.-7 cm/cm.degree.C. or so, they are employable for sealing leading wires made of an alloy of Fe-Ni-Co system, and as the inner layer of a boro silicate glass. These glasses are characterized by high contents of B.sub.2 O.sub.3 and MgO, while the CaO content is relatively low.
In another aspect of the prior art, fundamental studies of bone china have been undertaken. P. D. S. St. Pierre investigated the phase diagram of the three-component system of Ca.sub.3 (PO.sub.4).sub.2 -Al.sub.2 O.sub.3 -SiO.sub.2 at high temperatures, and reported the results of investigations in Journal of American Ceramics Society, Volume 37 (6), pages 243-253 (1954). Therein, glass pieces were obtained by charging a glass batch in an amount corresponding to 4 g of glass in a 10 ml of platinum crucible and then, by melting the glass batch at 1700.degree. C. for 20-30 minutes and next, by cooling the molten glass rapidly in water and further repeating the melting-rapid cooling procedure three times. The refractive indexes of the thus-obtained glasses were measured by the immersion method using a petrographic microscope, and the isofract diagram of the aforesaid three-component system was illustrated in a form of triangular composition diagram. On the basis of the investigation, St. Pierre reported that transparent and homogeneous glasses in the central part of the phase diagram of the three-component system were difficult to make because of their markedly high viscosities, as can be seen from the above-described procedures, and, furthermore, in both the high content range of SiO.sub.2 and Al.sub.2 O.sub.3, the vitrification of the system is nearly impossible, particularly using a platinum crucible. In addition, crystal species separated out of the solid-liquid phase boundary and the liquid phase of the pseudo three-component system of the above-described type (that is, the four-oxide component system of St. Pierre), as is summarized on the triangular composition diagram. As the summary shows, the liquidus temperatures are generally high and that the viscosities are also high. Therefore, it is difficult using such a system to make glass on the industrial scale.
In recent years, the advantages of ceramics and glasses or glass-ceramic materials have become appreciated as biological implanting materials, in the place of conventionally used plastic and metallic materials. Under these circumstances, studies of these materials have been made not only with the intention of solving the problems that the conventionally used implanting materials are sometimes toxic to or rejected by human bodies, but also with the positive intention of developing novel materials having such a biological replacing reactivity that they may adhere completely to some substances as implanting elements and that, after some definite period of time they may be replaced by living bones not only at the adhesion surfaces but also over the inner parts thereof.
Hydroxy apatite is supposed to be an inorganic material formed from the bones of formerly living bodies and contains calcium phosphate as a main component. Accordingly, calcium orthophosphate or hydroxy apatite powder has been used as a starting material for making implanting ceramic materials and it has been usually subjected to, in sequence, a pressure molding treatment and a sintering treatment (or the like) in order to make ceramic materials. In such a process, various attempts have been made to enhance the mechanical strength and to improve general physical properties such as pore characteristics, and what sorts of additives, what pressure applying means and what sintering conditions were preferable for the above-described purposes have been reported.
However, it appears that insofar as the method of making ceramic materials is based upon the powder sintering method, there is a limit to the physical and chemical homogeneity of the ceramic materials made. Microscopic homogeneity of the structure is, nevertheless, a very important essential item to an implanting material, as well as the macroscopic mechanical strength thereof. Further, these characteristics must be controlled adequately in the process of making the implanting material. Although various bioglasses and glass-ceramics have been proposed as materials capable of realizing the above-described aims, the compositions of those which are reported in the above-described, Japanese Patent Application (OPI) No. 73019/76 and Japanese Patent Publication 8970/76 have small contents of calcium phosphate, which is supposed to facilitate the biological replacement properties, or the glasses made from such compositions are chemically unstable. Therefore, glass-ceramics cannot be made from these compositions on a large scale.